1. Field of the Invention
This invention relates to the field of electromagnetic shielding, including conductive seals and gaskets for sealing between abutted parts of conductive bodies, and also to shielding material in sheet form, applied as a barrier to electromagnetic radiation passing into or out of enclosures, cables, conduits and the like. In particular, the invention concerns such shielding seals, gaskets and/or sheets wherein the material of the shield is affixed to a conductive surface by a discontinuous and preferably nonconductive adhesive.
2. Prior Art
Shielding against electromagnetic interference (EMI) involves providing a conductive barrier in which currents induced by incident electromagnetic fields are grounded and/or dissipated in eddy currents. Conductive seals and gaskets typically are used to render continuously conductive the junction of an abutment between conductive parts of the enclosures of electrical or electronic equipment. The conductive parts of an enclosure may be relatively movable, as in the case of a door of a cabinet, or the conductive parts can be intended to remain abutted, as in the case of panels which are held together by fasteners. The shielding of cables and the like typically involves surrounding the entire cable in a shield material Similarly, conductive seals and gaskets may have a conductive sheet material wrapped or wound on a compressible form. While it is possible to form a seamless tube of conductive sheet material for a cable shield, it is also possible to wrap the cable with conductive sheet material, defining a longitudinal or helical seam at which the conductive material must be joined so as to conduct across the seam. Seams are also defined in use between conductive material of a seal or gasket and the cabinet panels or the like to which the seal or gasket is affixed.
The shielding may be intended either to confine or to exclude electromagnetic interference. In connection with microwave ovens and the like, for example, the objective is to confine the microwave field to the inside of the enclosure. In connection with communications equipment and the like, the objective may be either or both of isolating the circuitry within an enclosure from ambient electromagnetic interference (EMI) and/or protecting other equipment from interference generated in the shielded equipment. Shielding can protect against potential damage as well as potential improper operation due to induction of currents by external fields. Apart from interference, electromagnetic shielding techniques may be employed to protect vulnerable circuitry from damage due to electromagnetic pulses, such as are produced by nuclear detonations.
Whether the plane or surface to be shielded is between movable or immovable conductive surfaces, or whether the shield material is to be wrapped around an element without bridging across conductive surfaces other than the shield material itself, the need to conductively fix the shield material at least at one seam is common to all shielding arrangements. In each case the shield resides across the path of incident electromagnetic radiation, defining a substantially continuously conductive body (either alone or in conduction with conductive panels or the like), so as to block propagation of electromagnetic fields.
A conductive gasket or seal is known wherein a conductive sheath of woven or knitted wire encloses a compressible core. This form of seal can be mounted, for example, in a slot in a first conductive panel or body and arranged to bear against a second panel or body brought into abutment with the first. In movable panel arrangements, one of the panels normally carries the seal and the other of the panels simply abuts against the seal. If the sealed panels are to remain immovable, the seal can be attached to one or both.
Typically, such a seal is attached to the conductive panel or the like at least partly by an adhesive at the connecting seam. For maximum shielding efficiency there is a need to avoid undue electrical resistance across the seam, i.e., between the seal and the surface against which it abuts. The resistance normally includes electrical resistance due to the adhesive disposed between the conductive panel and the conductive material of the seal. The typical technique for attaching the seal to the conductive panel is to place a continuous bead of conductive adhesive on the conductive sheath of the seal, at least on one side of the seal to be disposed against a conductive panel. Conductive adhesive is rendered conductive by conductive particles disposed in the adhesive binder, defining a conductive path through the adhesive due to surface contact of the particles within the binder. The binder typically consists essentially of a nonconductive elastomer which otherwise would function as an insulator, or in connection with adhesive disposed between conductive panels, as a dielectric.
Conductive adhesives tend to break down over time The conductive particles can migrate in the elastomeric binder, particularly with compression and decompression of the seal. Often the result of cycles of compression and decompression is that the conductive particles tend to become spaced from the surface of the adhesive as the elastomer flows viscously around the particles. This causes a gradual decrease in the conductivity of the seal as a whole, due to the increase in resistance across the adhesive. Additionally, the conductive particles in the binder of the adhesive can break down over time due to mechanical and environmental effects. The breakdown of the conductive portion of the adhesive is accelerated where the conductive sheath of the seal is made of a wire mesh or the like, wherein movement of the wire portions of the sheath relative to the viscous binder of the adhesive kneads the adhesive material
In U.S. Pat. No. 4,857,668--Buonanno, a seal is disclosed which includes a conductive sheath on a resilient foamed core. A preferred material for the sheath is ripstop nylon, a polymer fabric, and the filaments of the fabric can be plated with a conductive material (e.g., metal). The sheath can be mounted to one of the sealed panels by means of conductive clips or by engagement in a groove or the like. Alternatively, the seal can be attached to its panel via a conductive adhesive. The sheath according to Buonanno can be rendered conductive at least partly by an outer conductive coating of relatively inert particles such as carbon black in a binder of elastomer such as urethane. The conductive particles provide a current path, while the elastomeric binder as well as the inert conductive particles avoid presenting an abrasive surface or a surface subject to corrosion. Any corrosion on the surfaces in contact between a conductive seal and a conductive panel tends to reduce the smoothness of the seal, thereby presenting a possibility of gaps, and also tends to increase surface resistance, leading to reduced shielding effectiveness of the seal. The particular sheath composition is an important consideration and is given substantial attention by those skilled in the art.
Other forms of seals are also known for placement between conductive panels. The seals range from wholly metallic sheet metal structures, for example with spring-like metal forms protruding from a metal strip, or helically wound sheet metal strips, to resilient forms of rubber or plastic enclosed in woven, knitted or unwoven batts of conductive sheathing. In each case the seal provides conductive surfaces to be placed in contact with the conductive panels to be bridged by a conductive material for effecting an EMI shield. All these forms must be attached to the conductive panels, typically by an adhesive.
A seal formed in part of a sheet of conductive sheathing can be wrapped around a resilient core to form a seal or gasket to reside between conductive panels. In the same manner, a conductive sheath can be wrapped around a cable or the like. The sheathing may be formed as a tube, however, wrapping is more convenient. When wrapping an elongated strip around a core, cable or other elongated structure, the lateral edges of the sheathing must be conductively attached together along a longitudinal seam to ensure that the entire sheath is uniformly conductive. Similarly, when applying a conductive sheath (without a core) to a surface to be shielded, the sheath must be affixed to the surface both conductively and in a manner that physically retains the sheath in place. The physical retention of the sheath often requires an adhesive.
Assuming the conductive sheath or seal is to be attached (to itself or to conductive panels) in a routine manner with the object of improving the continuity and decreasing the resistance of the conductive path across the junction of the attachment, a conductive adhesive will provide a continuous conductive path. However, as noted above, conductive adhesives tend to break down and become less conductive over time. The inherent resistance of the adhesive is interspersed between the conductive elements at the junction, thereby increasing electrical resistance along the conductive path between the sealed panels. Conductive adhesive is more expensive than nonconductive adhesive due to the need for the additional conductive particles. The added particles render the adhesive less sticky than a comparable quantity of nonconductive adhesive due to the fact that the conductive particles displace the sticky adhesive material, thus requiring a greater quantity of adhesive for a given strength joint. For all these reasons, it would be helpful to avoid or minimize reliance on conductive adhesive for the electrical and/or mechanical joining of the parts of conductive sheaths and panels.
According to the present invention, the conventional use of conductive adhesive applied as a continuous bead or surface has been reconsidered. Rather than mounting a conductive sheathed seal body on a continuous strip of conductive adhesive, and rather than attaching a conductive sheet either at its edges or to a full surface, adhesive is applied discontinuously, for example in the form of regularly spaced adhesive dots, or preferably along spaced lines, at the conductive and physical junction of the sheath or seal. Contrary to expectations, the lack of an adhesive material in the area between the adhesive lines or dots does not reduce the effectiveness of the seal, particularly if the adhesive is arranged in the form of longitudinally non-overlapping lines inclined laterally across the connecting seam of the seal. In fact the seal is more effective because greater direct contact between the panels and the conductive sheath of the seal provides an overall conductive EMI shielding barrier that intersperses only the contact resistance of the sheath and the panel between the abutting elements along the path of electromagnetic propagation. While conductive adhesive can be used, a less expensive nonconductive adhesive is preferred, because the direct contact between the surface of the conductive sheath and the panel provides at least as good continuity along the length of the junction to compare favorably with conductive junctions having a conductive adhesive disposed continuously between the sheath and the panel, or at overlapped sections of the sheath. Moreover, the adhesion of the seal material at the seam is improved over conductive adhesives of equal amount, and the conductivity of the seal does not deteriorate over its lifetime.